This invention relates generally to a filter media, and more specifically to a high efficiency, high dirt holding capacity filter for automatic transmissions for automobiles.
Automatic transmissions in automobiles have filters that remove contaminant that is generated by or becomes entrained in the transmission system. These filters use a 100% polyester needle punch felt that is saturated with phenolic resin. The xe2x80x9cBig 3xe2x80x9d OEM""s (GM, Ford, and Chrysler) specify for original equipment applications a material that must be 90% efficient at removing 60 micron diameter particles or larger with a low clean pressure drop. ISO 4572 (Multi-pass Filtration Performance Testxe2x80x94Beta) is used as the hydraulic evaluation method to determine filter performance. A flat-sheet version of this test is used in evaluating filter media.
Recently designed transmissions have smaller clearances and are susceptible to smaller contaminant particles in the oil. Specifically, 40 micron diameter particles in the transmission fluid have been found to foul the electronic valves in the transmission. The OEM""s want to change their specifications to make the transmission filter more efficient at removing these particles without any loss in filter life or increase in clean pressure drop across the filter, corresponding to a loss in gas mileage due to increased power consumption. Current filters are between 40-75% efficient for removing 40 micron diameter particles or larger. The OEM""s have asked for at least 80% efficiency at 40 microns diameter or larger and desire 90% efficiency at 40 microns diameter or larger and 99% efficiency at 60 microns diameter or larger with no loss in dirt holding capacity or increase in clean pressure drop.
A number of companies have attempted to achieve these performance levels but have been unsuccessful. Single-layered felts have been xe2x80x9ctightenedxe2x80x9d by increasing their density and decreasing their permeability to achieve close to the level of efficiency required. This has resulted in unacceptable increases in clean pressure drop, and losses in dirt holding capacity in the filter element. Saturated felts with precision woven 70-75 micron diameter hole polyester screens laminated to them have been developed that are 70-75% efficient at 40 microns diameter or larger, with no appreciable increase in pressure drop but with dirt holding capacities equivalent to the single layered felt filter media. U.S. Pat. No. 3,784,011 teaches the addition of a second layer of felt to the filter to reduce the clean pressure drop by presenting a low resistance to fluid flow. The filter materials work in parallel as the flow of transmission fluid passes through either one or the other felt layers, but does not pass through both. The low resistance to flow is due to the additional filter media area in the filter. U.S. Pat. No. 4,093,437 describes a multilayered air filter material which is improved in dust holding capacity and in air filter efficiency. This patent teaches that the third layer of the construction is produced in such a manner as to xe2x80x9ccatch fine particles passed through the upper layer and the intermediate layerxe2x80x9d. This tight bottom layer gives the filter material a high clean pressure drop and high starting resistance to flow before it starts to load with dust as compared to the other filter materials disclosed in the patent. U.S. Pat. No. 4,701,197 describes a molded air panel filter of two different members. One of the members is molded into a convoluted design of substantially parallel pleats with the other filter member being an open, xe2x80x9cbatt-likexe2x80x9d material especially adapted to capturing fine particles, both members requiring thermoplastic fibers to be moldable into the filter article. This filter is not suitable for a transmission filter as this media construction would not withstand the operating temperature of the transmission environment.
U.S. Pat. No. 5,728,298 teaches a molded gradient density filter element with an inner shell of a first large pore size porous media adjacent the hollow core (downstream to fluid flow) and forming a majority of the filter structure, and an outer shell of a second small pore size porous media (upstream to fluid flow), finer than the first porous media and adjacent to the inner shell. The first and second porous media are resin impregnated and bonded together. The filter is described as having finer efficiency than if the finer porous media were on the downstream side adjacent the hollow inner tube. The filtering technique that is employed by this filter design (a molded filter cylinder) is surface filtration. Therefore, the area of fine pore surface available for filtering is key to how the filter performs. A surface filter of this type will have less available area on the inside diameter of the hollow core than on the outside of the filter. If the fluid volumetric flow is constant through the filter, then the filter will have a much lower face velocity (defined as the volume of fluid divided by the available fine pore size filter surface area) when the fine pore size surface is on the outside of the filter then if the fine pore size surface is the same as the hollow core inside diameter of the filter. It is well known in the art of filtration that a significant increase in efficiency is to be expected when the face velocity of the fluid flow across the filtering surface is reduced significantly, as is with the filter of the ""298 patent. It is also well known in the art of filtration that, when surface filtration is the predominant filtering technique employed in the filter, the dirt holding capacity of the filter (or amount of time it will take for the surface of the filter to plug) is reduced when the fine surface of the filter medium is the first to see fluid flow. In this case, the filter efficiency was increased by increasing the area of the fine pore surface media at the expense of the dirt holding capacity of the filter.
U.S. Pat. No. 5,288,402 is directed to a liquid filter medium including a fibrillated filtering layer and an organic fiber support. The patent teaches the use of fine fibers (blends of 1 micron and below, 1-5 microns, and greater than 5 microns) in a two layer filter medium. It also uses a binder fiber as a part of the mix which cannot be used in a transmission filter. The ""402 patent does not describe the thickness of the filter material and does not appear concerned with void volume.
It is, therefore, an object of the present invention to provide a filter medium which overcomes the problems of prior art described above.
It is a further object of the present invention to provide a transmission filter medium with a dirt removal efficiency of at least 99% for particles 60 microns diameter and larger.
It is another object of the present invention to provide a transmission filter medium with a dirt removal efficiency of at least 80% for particles of 40 microns diameter and larger.
It is yet another object of the present invention to provide a highly efficient filter medium which exhibits superior dirt holding capacity.
It is yet a further object of the present invention to provide a highly efficient filter medium which exhibits no significant increase in clean pressure drop.
It is yet another object of the present invention to provide a transmission filter that is highly efficient, exhibits high dirt holding capacity, and exhibits a low clean pressure drop.
These and other objects are achieved, as described herein, by forming individual batts of fibers from selected fiber types and fiber fineness, which are blended in selected ratios to achieve a desired length averaged fiber diameter for the desired gradient density final composite felt material. This is followed by consolidating the fiber batts through needling into individual felt components and assembling at least two single fiber felt components into a composite felt material, combining the individual felts together in series into a single gradient density felt that has been needled together into a composite felt material having a more open, fine particle removing fiber component upstream and a coarse particle removing fiber component downstream. The composite felt is constructed to achieve the desired final length averaged fiber diameter. The composite felt is then singed to remove loose fibers, and then is saturated with a specified amount of a resin to form the final filter material.
As described in a paper presented at the Second World Filtration Congress 1979 entitled xe2x80x9cFilter Media Performance and Fibre Morphologyxe2x80x9d by C. A. Rodman, which is incorporated herein by reference, the present invention works by using the impaction mechanism of filtration to selectively remove the smaller, plugging particles from the fluid stream before a filter cake can build and be plugged by them. The openness of the pore structure of the top portion of the layer of the gradient density material allows the larger particles to move by inertia through this portion of the felt, and become mechanically entrapped by the tighter downstream portion of the felt. The removal of the fine particles first and the coarser particles second means that the gradient density structure acts as a particle classifier, selectively removing particles of small size upstream so that the larger particles can be sieved downstream. This allows the filter cake to build longer with large particles without significant increases in pressure drop, and create a more porous filter cake and longer useful life.